Thorium-to-Uranium Ratios as Indicators of Sedimentary Processes: Example of Concept of Geochemical Facies

Abstract

Because of analytical difficulties, few data are available on the thorium and uranium contents of sedimentary rocks. More than 200 new thorium and uranium determinations have been made by gamma-ray spectral technique and by an alpha activity-fluorometric uranium technique. Together these two independent techniques can be used as an experimental test of secular radioactive equilibrium. Only rarely in this study have fresh samples of ancient sedimentary rocks been found out of radioactive equilibrium. The accuracy of the thorium-to-uranium ratio determinations is more than sufficient for many geologic studies. The thorium-to-uranium ratios in sedimentary rocks range from less than 0.02 to more than 21. Ratios in many oxidized continental deposits are above 7, whereas most marine deposits have ratios much below 7. Thus, the thorium-to-uranium ratio varies with sedimentary processing and depositional environment. A cyclothem and several other sedimentary sequences illustrate the use of this ratio to distinguish environments and processes. The thorium content of shales varies much less than the uranium content. By mineral and trace-element analysis an attempt has been made to evaluate the resistate, hydrolyzate, clay, and precipitate (evaporite) contributions to the thorium and uranium contents of sedimentary rocks. These data also provide some insight into the details of the mobilization, tra sportation, and fixation of thorium and uranium in the sedimentary cycle. Field tests indicate that quantitative potassium, uranium, and thorium determinations can be made with a spectral gamma-ray logging instrument. Logs obtained with such instruments may provide an important additional means for subsurface interpretations.